Stacked bulk acoustic resonator band-pass filter with controllable pass bandwidth

ABSTRACT

The band-pass filter has a stacked pair of film bulk acoustic resonators (FBARs) and an acoustic decoupler between the FBARs. Each of the FBARs has opposed planar electrodes and a layer of piezoelectric material between the electrodes. The acoustic decoupler controls the coupling of acoustic energy between the FBARs. Specifically, the acoustic decoupler couples less acoustic energy between the FBARs than would be coupled by direct contact between the FBARs. The reduced acoustic coupling gives the band-pass filter desirable in-band and out-of-band properties.

RELATED APPLICATION

This application is related to U.S. patent application Ser. No.10/XXX,XXX of John D. Larson III and Richard Ruby entitled Thin-FilmAcoustically-Coupled Transfonner (Agilent Docket No. 10030993), filed onthe filing date of this application and incorporated into thisapplication by reference.

BACKGROUND

Electrical band-pass filters are used in many different types ofconsumer and industrial electronic product to select or rejectelectrical signals in a range of frequencies. In recent years, thephysical size of such products has tended to decrease significantlywhile the circuit complexity of the products has tended to increase.Consequently, a need for highly miniaturized, high-performance band-passfilters exists. A special need for such band-pass filters exists incellular telephones in which the antenna is connected to the output ofthe transmitter and the input of the receiver through a duplexer thatincludes two band-pass filters.

Modern cellular telephones incorporate a duplexer in which each of theband-pass filters includes a ladder circuit in which each element of theladder circuit is a film bulk acoustic resonator (FBAR). Such a duplexeris disclosed by Bradley et al. in U.S. Pat. No. 6,262,637 entitledDuplexer Incorporating Thin-film Bulk Acoustic Resonators (FBARs),assigned to the assignee of this disclosure and incorporated into thisdisclosure by reference. Such duplexer is composed of a transmitterband-pass filter connected in series between the output of thetransmitter and the antenna and a receiver band-pass filter connected inseries with 90° phase-shifter between the antenna and the input of thereceiver. The center frequencies of the pass-bands of the transmitterband-pass filter and the receiver band-pass filter are offset from oneanother.

FIG. 1 shows an exemplary embodiment of an FBAR-based band-pass filter10 suitable for use as the transmitter band-pass filter of a duplexer.The transmitter band-pass filter is composed of series FBARs 12 andshunt FBARs 14 connected in a ladder circuit. Series FBARs 12 have ahigher resonant frequency than shunt FBARs 14.

FBARs are disclosed by Ruby et al. in U.S. Pat. No. 5,587,620 entitledTunable Thin Film Acoustic Resonators and Method of Making Same, nowassigned to the assignee of this disclosure and incorporated into thisdisclosure by reference. FIG. 2 shows an exemplary embodiment 20 of anFBAR. FBAR 20 is composed a pair of electrodes 24 and 26 and a layer ofpiezoelectric material 22 sandwiched between the electrodes. Thepiezoelectric material and electrodes are suspended over a cavity 28defined in a substrate 30. This way of suspending the FBAR allows theFBAR to resonate mechanically in response to an electrical signalapplied between the electrodes. Other suspension schemes that allow theFBAR to resonate mechanically are possible.

Also disclosed in the above-mentioned U.S. Pat. No. 5,587,620 is astacked thin-film bulk acoustic resonator (SBAR). FIG. 3 shows anexemplary embodiment 40 of the SBAR disclosed in U.S. Pat. No.5,587,620. SBAR 40 is composed of two layers 22, 42 of piezoelectricmaterial interleaved with three electrodes 24, 26, 44. An inputelectrical signal is applied between electrodes 44 and 26 and an outputelectrical signal is provided between electrodes 24 and 26. The centerelectrode 26 is common to both the input and the output.

The SBAR disclosed in U.S. Pat. No. 5,587,620 was thought to havepromise for use as a band-pass filter because it has an inherentband-pass characteristic. However, practical examples of the SBARexhibit an extremely narrow pass bandwidth that makes the SBARunsuitable for use in most band-pass filtering applications, includingthe cellular telephone duplexer application referred to above. Thenarrow pass bandwidth of the SBAR can be seen in FIG. 4, which comparesthe frequency response of a practical example of SBAR 40 shown in FIG. 3(curve 46) with the frequency response a practical example of theFBAR-based band-pass ladder filter shown in FIG. 1 (curve 48). FIG. 4also shows that, while the frequency response of the ladder filter shownin FIG. 1 advantageously falls sharply outside the pass-band, as thefrequency difference from the center frequency further increases, thefrequency response undesirably rises again.

What is needed, therefore, is a band-pass filter with a low insertionloss and flat frequency response in its pass band, a pass bandwidth inthe range from about 3% to about 5% of a center frequency anywhere fromabout 0.5 GHz to about 10 GHz and good out-of-band rejection. What isalso needed is such a band-pass filter with the structural simplicity ofthe SBAR.

SUMMARY OF THE INVENTION

The invention provides in a first aspect a band-pass filter that has astacked pair of film bulk acoustic resonators (FBARs) and an acousticdecoupler between the FBARs. Each of the FBARs has opposed planarelectrodes and a layer of piezoelectric material between the electrodes.The acoustic decoupler controls the coupling of acoustic energy betweenthe FBARs. Specifically, the acoustic decoupler couples less acousticenergy between the FBARs than would be coupled by direct contact betweenthe FBARs as in the exemplary SBAR shown in FIG. 3. The reduced acousticcoupling gives the band-pass filter such desirable properties as a lowinsertion loss and flat frequency response in its pass band, a passbandwidth in the range from about 3% to about 5% of the center frequencyand good out-of-band rejection.

In one embodiment, the acoustic decoupler includes a layer of acousticdecoupling material having an acoustic impedance less than that of theother materials of the FBARs. In another embodiment, the acousticdecoupler includes a Bragg structure.

In another aspect, the invention provides a band-pass filtercharacterized by a center frequency. The band-pass filter has a stackedpair of film bulk acoustic resonators (FBARs) and a layer of acousticdecoupling material between the FBARs. Each of the FBARs has opposedplanar electrodes and a layer of piezoelectric material between theelectrodes. The layer of acoustic decoupling material has a nominalthickness equal to an odd integral multiple of one quarter of thewavelength in the acoustic decoupling material of an acoustic wavehaving a frequency equal to the center frequency. The acousticdecoupling material has an acoustic impedance less than the acousticimpedance of the piezoelectric material.

In another aspect, the invention provides an electrical filteringmethod. In the method, a pair of film bulk acoustic resonators (FBARs)is provided. An input electrical signal is applied to one of the FBARs.Acoustic energy is coupled between the FBARs. The acoustic energycoupled is less than would be coupled by direct contact between theFBARs. A filtered output electrical signal is output from the other ofthe FBARs.

In a final aspect, the invention provides a method of fabricating anacoustically-coupled device. In the method, a first film bulk acousticresonator (FBAR) is fabricated and an acoustic decoupler is fabricatedon the first FBAR. A second FBAR is fabricated on the acousticdecoupler. Fabricating the second FBAR on the acoustic decouplerinvolves subjecting the acoustic decoupler to a maximum temperature.Prior to fabricating the second FBAR, the first FBAR and the acousticdecoupler are baked at a temperature not lower than the maximumtemperature. This ensures a reliable bond between the second FBAR andthe acoustic decoupler.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of a band-pass filter incorporating FBARs.

FIG. 2 is a schematic side view of an FBAR.

FIG. 3 is a schematic side view of an SBAR.

FIG. 4 is a graph comparing the calculated frequency response of theSBAR shown in FIG. 3 and that of the FBAR-based band-pass filter shownin FIG. 1.

FIG. 5A is a plan view of an example of a first embodiment of aband-pass filter in accordance with the invention.

FIG. 5B is a cross-sectional view of the band-pass filter shown in FIG.5A along the section line 5B-5B.

FIG. 5C is an enlarged cross-sectional view of part of the band-passfilter shown in FIG. 5A along the section line 5B-5B showing a firstembodiment of the acoustic decoupler.

FIG. 5D is an enlarged cross-sectional view of part of the band-passfilter shown in FIG. 5A along the section line 5B-5B showing a secondembodiment of the acoustic decoupler.

FIG. 6 is a graph comparing the calculated frequency responses ofembodiments of the band-pass filter in accordance with the inventionincorporating acoustic decouplers of acoustic decoupling materialshaving different acoustic impedances.

FIGS. 7A-7J are plan views illustrating a process for making a band-passfilter in accordance with the invention.

FIGS. 7K-7S are cross-sectional views along the section lines 7K-7K,7L-7L, 7M-7M, 7N-7N, 7O-7O, 7P-7P, 7Q-7Q, 7R-7R, 7S-7S and 7T-7T inFIGS. 7A-7J, respectively.

FIG. 8 is a schematic drawing of an example of a second embodiment of aband-pass filter in accordance with the invention.

FIG. 9 is a graph comparing the calculated frequency response of theembodiment of the band-pass filter shown in FIG. 8 with the embodimentof the band-pass filter shown in FIGS. 5A and 5B.

DETAILED DESCRIPTION

The SBAR shown in FIG. 3 can be regarded as being composed of two FBARs,one stacked on top of the other. One of the FBARs is composed ofpiezoelectric layer 22 sandwiched between electrodes 24 and 26. Theother of the FBARs is composed of piezoelectric layer 42 sandwichedbetween electrodes 26 and 44. Electrode 26 common to both FBARs providesclose coupling of acoustic energy between the FBARs. This results in theFBARs being acoustically highly over-coupled so that SBAR 40 exhibitsthe single Lorentzian resonance illustrated in curve 46 of FIG. 4. Thesingle Lorentzian resonance makes it difficult or impossible to design aband-pass filter with such desirable characteristics such as broad passband, a flat in-band frequency response and a sharp roll-off outside thepass band.

FIG. 5A is a schematic side view showing the structure of an exemplaryembodiment 100 of a band-pass filter in accordance with the invention.FIG. 5B is a cross-sectional view along the section line 5B-5B in FIG.5A. Band-pass filter 100 is composed of a stacked pair of film bulkacoustic resonators (FBARs) 110 and 120. In the example shown, FBAR 120is stacked atop FBAR 110. FBAR 110 is composed of opposed planarelectrodes 112 and 114 and a layer 116 of piezoelectric material betweenthe electrodes. FBAR 120 is composed of opposed planar electrodes 122and 124 and a layer 126 of piezoelectric material between theelectrodes. Band-pass filter 100 is also composed of an acousticdecoupler 130 between FBARs 110 and 120, specifically, betweenelectrodes 114 and 122. The acoustic decoupler controls the coupling ofacoustic energy between FBARs 110 and 120. Specifically, the acousticdecoupler couples less acoustic energy between the FBARs than would becoupled by direct contact between the FBARs as in the exemplary SBARshown in FIG. 3.

In the example shown, the stacked FBARs 110 and 120 are suspended over acavity 104 defined in a substrate 102. This way of suspending thestacked FBARs allows the stacked FBARs to resonate mechanically inresponse to an input electrical signal applied between the electrodes ofone of them. Other suspension schemes that allow the stacked FBARs toresonate mechanically in response to an input electrical signal arepossible. For example, the stacked FBARs can be located over amismatched acoustic Bragg reflector (not shown) formed in or onsubstrate 102, as disclosed by Lakin in U.S. Pat. No. 6,107,721, thedisclosure of which is incorporated into this disclosure by reference.

As noted above, acoustic decoupler 130 controls the acoustic couplingbetween FBARs 110 and 120. The acoustic coupling provided by acousticdecoupler 130 is substantially less than the acoustic coupling betweenthe FBARs in the SBAR embodiment shown in FIG. 3. As a result, FBARs 110and 120 are not over coupled, and band-pass filter 100 has a relativelybroad and flat in-band response and a sharp roll-off outside the passband instead of the single Lorentzian resonance shown in FIG. 4 (curve46) of the over-coupled conventional SBAR. The frequency response ofband-pass filter 100 will be described further below with reference toFIG. 6.

FIG. 5C is an enlarged view of a first embodiment of acoustic decoupler130 in which the acoustic decoupler is composed of a layer 131 ofacoustic decoupling material located between the electrodes 114 and 122of FBARs 110 and 120, respectively (FIG. 5B). Layer 131 of acousticdecoupling material has a nominal thickness that is an odd integralmultiple of one quarter of the wavelength in the acoustic decouplingmaterial of an acoustic wave having a frequency equal to the centerfrequency of band-pass filter 100. The acoustic decoupling material hasan acoustic impedance less than that of the piezoelectric material thatconstitutes the FBARs 110, 120. In embodiments of band-pass filter 100that additionally provide electrical isolation between input and output,the acoustic decoupling material additionally has a high electricalresistivity and a low dielectric permittivity.

As noted above, the acoustic decoupling material of acoustic decoupler130 has an acoustic impedance less that of the piezoelectric material ofFBARs 110 and 120. The acoustic decoupling material also has an acousticimpedance substantially greater than that of air. The acoustic impedanceof a material is the ratio of stress to particle velocity in thematerial and is measured in Rayleighs, abbreviated as rayl. Thepiezoelectric material of layers 116, 216 of the FBARs is typicallyaluminum nitride (AIN). The acoustic impedance of AIN is typically about35 Mrayl and that of molybdenum, a typical electrode material, is about63 Mrayl. The acoustic impedance of air is about 1 krayl. In embodimentsof band-pass filter 100 in which the materials of FBARs 110, 120 are asstated above, materials with an acoustic impedance in the range fromabout 2 Mrayl to about 16 Mrayl work well as the acoustic decouplingmaterial of layer 131.

FIG. 6 shows how the calculated frequency response of band-pass filter100 depends on the acoustic impedance of the acoustic decouplingmaterial of layer 131 that constitutes an embodiment of acousticdecoupler 130. The embodiment illustrated has a center frequency ofabout 1,900 MHz. Calculated frequency responses for embodiments in whichthe acoustic decoupling material has an acoustic impedance of about 4Mrayl, e.g., polyimide, (curve 140), about 8 Mrayl (curve 142) and about16 Mrayl (curve 144) are shown. It can be seen that the width of thepassband of the band-pass filter increases with increasing acousticimpedance of the acoustic decoupling material. Accordingly, by making anappropriate choice of the acoustic decoupling material, embodiments ofband-pass filter 100 having a desired pass-band characteristic can bemade.

The embodiment in which the acoustic decoupling material of layer 131 ispolyimide (curve 140) exhibits some under coupling of acoustic energybetween FBARs 110, 120, but nevertheless has a pass band that isusefully wide. The embodiment in which the acoustic decoupling materialhas an acoustic impedance of about 8 Mrayl (curve 142) exhibits nearcritical coupling of acoustic energy between FBARs 110, 120. Theembodiment in which the acoustic impedance of the acoustic decouplingmaterial is about 16 Mrayl (curve 144) exhibits a double peak in thein-band response typical of significant over coupling of acoustic energybetween FBARs 110, 120. An embodiment in which the acoustic decouplingmaterial had an acoustic impedance intermediate between 4 Mrayl and 8Mrayl would have an in-band response that included a flat portionindicative of critical coupling of acoustic energy between FBARs 110,120. FIG. 6 also shows that embodiments in which the acoustic decouplingmaterial has an acoustic impedance of 8 Mrayl or less have an insertionloss of less than 3 dB, and some embodiments have an insertion loss ofless than 1 dB.

The embodiment of acoustic decoupler 130 shown in FIG. 5C is composed oflayer 131 of acoustic decoupling material with a nominal thickness equalto one quarter of the wavelength in the acoustic decoupling material ofan acoustic wave having a frequency equal to the center frequency of theband-pass filter, i.e., t≈λ_(n)/4, where t is the thickness of layer 131and λ_(n) is the wavelength in the acoustic decoupling material of anacoustic wave having a frequency equal to the center frequency ofband-pass filter 100. A thickness of layer 131 within approximately ±10%of the nominal thickness can alternatively be used. A thickness outsidethis range can alternatively be used with some degradation inperformance. However, the thickness of layer 131 should differsignificantly from 0λ_(n) at one extreme (see FIG. 3) and λ_(n)/2 at theother extreme.

More generally, the embodiment of acoustic decoupler 130 shown in FIG.5C is composed of layer 131 of acoustic decoupling material with anominal thickness equal to an odd integral multiple of one quarter ofthe wavelength in the acoustic decoupling material of an acoustic wavehaving a frequency equal to the center frequency of band-pass filter100, i.e., t≈(2m+1)λ_(n)/4, where t and λ_(n) are as defined above and mis an integer equal to or greater than zero. In this case, a thicknessof layer 131 that differs from the nominal thickness by approximately±10% of λ_(n)/4 can alternatively be used. A thickness tolerance outsidethis range can be used with some degradation in performance, but thethickness of layer 131 should differ significantly from an integralmultiple of λ_(n)/2.

In an embodiment of acoustic decoupler 130, layer 131 is formed by spincoating the acoustic decoupling material over electrode 114. A layerformed by spin coating will typically have regions of differentthickness due to the contouring of the surface coated by the acousticdecoupling material of layer 131. In such embodiment, the thickness oflayer 131 of acoustic decoupling material is the thickness of theportion of the layer located between electrodes 114 and 122.

Many plastic materials have acoustic impedances in the range statedabove and can be applied in layers of uniform thickness in the thicknessranges stated above. Such plastic materials are therefore potentiallysuitable for use as the acoustic decoupling material of layer 131 ofacoustic decoupler 130. However, the acoustic decoupling material mustalso be capable of withstanding the temperatures of the fabricationoperations performed after layer 131 of acoustic decoupling material hasbeen deposited on electrode 114 to form acoustic decoupler 130. As willbe described in more detail below, in practical embodiments of band-passfilter 100, electrodes 122 and 124 and piezoelectric layer 126 aredeposited by sputtering after layer 131 has been deposited. Temperaturesas high as 300° C. are reached during these deposition processes. Thus,a plastic that remains stable at such temperatures is used as theacoustic decoupling material.

Plastic materials typically have a very high acoustic attenuation perunit length compared with the other materials of FBARs 110 and 120.However, since the above-described embodiment of acoustic decoupler 130is composed of layer 131 of plastic acoustic decoupling materialtypically less than 1 μm thick, the acoustic attenuation introduced bylayer 131 of acoustic decoupling material is typically negligible.

In one embodiment, a polyimide is used as the acoustic decouplingmaterial of layer 131. Polyimide is sold under the trademark Kapton® byE. I. du Pont de Nemours and Company. In such embodiment, acousticdecoupler 130 is composed of layer 131 of polyimide applied to electrode114 by spin coating. Polyimide has an acoustic impedance of about 4Mrayl. In another embodiment, a poly(para-xylylene) is used as theacoustic decoupling material of layer 131. In such embodiment, acousticdecoupler 130 is composed of layer 131 of poly(para-xylylene) applied toelectrode 114 by vacuum deposition. Poly(para-xylylene) is also known inthe art as parylene. The dimer precursor di-para-xylylene from whichparylene is made and equipment for performing vacuum deposition oflayers of parylene are available from many suppliers. Parylene has anacoustic impedance of about 2.8 Mrayl.

In an alternative embodiment, the acoustic decoupling material of layer131 constituting acoustic decoupler 130 has an acoustic impedancesubstantially greater than the materials of FBARs 110 and 120. Nomaterials having this property are known at this time, but suchmaterials may become available in future, or lower acoustic impedanceFBAR materials may become available in future. The thickness of layer131 of such high acoustic impedance acoustic decoupling material is asdescribed above.

FIG. 5D is an enlarged view of part of band-pass filter 100 showing asecond embodiment of acoustic decoupler 130 that incorporates a Braggstructure 161. Bragg structure 161 is composed of a low acousticimpedance Bragg element 163 sandwiched between high acoustic impedanceBragg elements 165 and 167. Low acoustic impedance Bragg element 163 isa layer of a low acoustic impedance material whereas high acousticimpedance Bragg elements 165 and 167 are each a layer of high acousticimpedance material. The acoustic impedances of the materials of theBragg elements are characterized as “low” and “high” with respect to oneanother and with respect to the acoustic impedance of the piezoelectricmaterial of layers 116 and 126. In embodiments of band-pass filter 100that additionally provide electrical isolation between input and output,at least one of the Bragg elements additionally has a high electricalresistivity and a low dielectric permittivity.

Each of the layers constituting Bragg elements 161, 163 and 165 has anominal thickness equal to an odd integral multiple of one quarter ofthe wavelength in the material the layer of an acoustic wave having afrequency equal to the center frequency of band-pass filter 100. Layersthat differ from the nominal thickness by approximately ±10% of onequarter of the wavelength can alternatively be used. A thicknesstolerance outside this range can be used with some degradation inperformance, but the thickness of the layers should differ significantlyfrom an integral multiple of one-half of the wavelength.

In an embodiment, low acoustic impedance Bragg element 163 is a layer ofsilicon dioxide (SiO₂), which has an acoustic impedance of about 13Mrayl, and each of the high acoustic impedance Bragg elements 165 and167 is a layer of the same material as electrodes 114 and 122,respectively, i.e., molybdenum, which has an acoustic impedance of about63 Mrayl. Using the same material for high acoustic impedance Braggelements 165 and 167 and electrodes 114 and 122, respectively, of FBARs110 and 120, respectively (FIG. 5B), allows high acoustic impedanceBragg elements 165 and 167 additionally to serve as electrodes 114 and122, respectively.

In an example, high acoustic impedance Bragg elements 165 and 167 have athickness of one quarter of the wavelength in molybdenum of an acousticwave having a frequency equal to the center frequency of band-passfilter 100, and low acoustic impedance Bragg element 163 has a thicknessof three quarters of the wavelength in SiO₂ of an acoustic wave having afrequency equal to the center frequency of the band-pass filter. Using athree-quarter wavelength-thick layer of SiO₂ instead of a one-quarterwavelength thick layer of SiO₂ as low acoustic impedance Bragg element163 reduces the capacitance between FBARs 110 and 120.

In embodiments in which the acoustic impedance difference between highacoustic impedance Bragg elements 165 and 167 and low acoustic impedanceBragg element 163 is relatively low, Bragg structure 161 may be composedof more than one (e.g., n) low acoustic impedance Bragg elementinterleaved with a corresponding number (i.e., n+1) of high acousticimpedance Bragg elements. Only one of the Bragg elements need beinsulating. For example, the Bragg structure may be composed of two lowacoustic impedance Bragg elements interleaved with three high acousticimpedance Bragg elements.

Wafer-scale fabrication is used to fabricate thousands of band-passfilters similar to band-pass filter 100 at the same time. Suchwafer-scale fabrication makes the band-pass filters inexpensive tofabricate. An exemplary fabrication method will be described next withreference to the plan views of FIGS. 7A-7J and the cross-sectional viewsof FIGS. 7K-7T.

A wafer of single-crystal silicon is provided. A portion of the waferconstitutes, for each band-pass filter being fabricated, a substratecorresponding to the substrate 102 of band-pass filter 100. FIGS. 7A-7Jand FIGS. 7K-7T illustrate and the following description describes thefabrication of band-pass filter 100 in and on a portion of the wafer. Asband-pass filter 100 is fabricated, the remaining band-pass filters onthe wafer as similarly fabricated.

The portion of the wafer that constitutes substrate 102 of band-passfilter 100 is selectively wet etched to form cavity 104, as shown inFIGS. 7A and 7K.

A layer of fill material (not shown) is deposited on the surface of thewafer with a thickness sufficient to fill the cavities. The surface ofthe wafer is then planarized to leave the cavities filled with the fillmaterial. FIGS. 7B and 7L show cavity 104 in substrate 102 filled withfill material 105.

In an embodiment, the fill material was phosphosilicate glass (PSG) andwas deposited using conventional low-pressure chemical vapor deposition(LPCVD). The fill material may alternatively be deposited by sputtering,or by spin coating.

A layer of metal is deposited on the surface of the wafer and the fillmaterial. The metal is patterned to define electrode 112, a bonding pad132 and an electrical trace 133 extending between electrode 112 andbonding pad 132, as shown in FIGS. 7C and 7M. Electrode 112 typicallyhas an irregular shape in a plane parallel to the major surface of thewafer. An irregular shape minimizes lateral modes in the FBAR 110 ofwhich it forms part, as described in U.S. Pat. No. 6,215,375 of LarsonIII et al., the disclosure of which is incorporated into this disclosureby reference. Electrode 112 is shaped and located to expose part of thesurface of fill material 105 so that the fill material can later beremoved by etching, as will be described below.

The metal layers in which electrodes 112, 114, 122 and 124 (FIG. 5B) aredefined are patterned such that, in respective planes parallel to themajor surface of the wafer, electrodes 112 and 114 have the same shape,size, orientation and position, electrodes 122 and 124 have the sameshape, size, orientation and position, and electrodes 114 and 122typically have the same shape, size, orientation and position.

In an embodiment, the metal deposited to form electrode 112, bonding pad132 and trace 133 was molybdenum. The molybdenum was deposited with athickness of about 440 nm by sputtering, and was patterned by dryetching to define a pentagonal electrode with an area of about 26,000square μm. Other refractory metals such as tungsten, niobium andtitanium may alternatively be used as the material of electrode 112,bonding pad 132 and trace 133. The electrode, bonding pad and trace mayalternatively comprise layers of more than one material.

A layer of piezoelectric material is deposited and is patterned todefine piezoelectric layer 116 as shown in FIGS. 7D and 7N.Piezoelectric layer 116 is patterned to expose part of the surface offill material 105 and bonding pad 132 of electrode 112. Piezoelectriclayer 116 is additionally patterned to define windows 119 that provideaccess to additional parts of the surface of the fill material.

In an embodiment, the piezoelectric material deposited to formpiezoelectric layer 116 was aluminum nitride and was deposited with athickness of about 780 nm by sputtering. The piezoelectric material waspatterned by wet etching in potassium hydroxide or by chlorine-based dryetching. Alternative materials for piezoelectric layer 116 include zincoxide and lead zirconium titanate.

A layer of metal is deposited and is patterned to define electrode 114,a bonding pad 134 and an electrical trace 135 extending betweenelectrode 114 and bonding pad 134, as shown in FIGS. 7E and 7O.

In an embodiment, the metal deposited to form electrode 114 wasmolybdenum. The molybdenum was deposited with a thickness of about 440nm by sputtering, and was patterned by dry etching. Other refractorymetals may alternatively be used as the material of electrode 114,bonding pad 134 and trace 135. The electrode, bonding pad and trace mayalternatively comprise layers of more than one material.

A layer of acoustic decoupling material is then deposited and ispatterned to define acoustic decoupler 130, as shown in FIGS. 7F and 7P.Acoustic decoupler 130 is shaped to cover at least electrode 114, and isadditionally shaped to expose part of the surface of fill material 105and bonding pads 132 and 134. Acoustic decoupler 130 is additionallypatterned to define windows 119 that provide access to additional partsof the surface of the fill material.

In an embodiment, the acoustic decoupling material was polyimide with athickness of about 750 nm, i.e., three quarters of the center frequencywavelength in the polyimide. The polyimide was deposited by spincoating, and was patterned by photolithography. Polyimide isphotosensitive so that no photoresist is needed. As noted above, otherplastic materials can be used as the acoustic decoupling material. Theacoustic decoupling material can be deposited by methods other than spincoating.

In an embodiment in which the material of the acoustic decoupler 130 waspolyimide, after deposition and patterning of the polyimide, the waferwas baked at about 300° C. before further processing was performed. Thebake evaporates volatile constituents of the polyimide and prevents theevaporation of such volatile constituents during subsequent processingfrom causing subsequently-deposited layers to separate.

A layer of metal is deposited and is patterned to define electrode 122and an electrical trace 137 extending from electrode 122 to bonding pad134, as shown in FIGS. 7G and 7Q. Bonding pad 134 is also electricallyconnected to electrode 114 by trace 135.

In an embodiment, the metal deposited to form electrode 122 wasmolybdenum. The molybdenum was deposited with a thickness of about 440nm by sputtering, and was patterned by dry etching. Other refractorymetals may alternatively be used as the material of electrode 122 andtrace 137. The electrode and the trace may alternatively comprise layersof more than one material.

A layer of piezoelectric material is deposited and is patterned todefine piezoelectric layer 126. Piezoelectric layer 126 is shaped toexpose bonding pads 132 and 134 and to expose part of the surface offill material 105 as shown in FIGS. 7H and 7R. Piezoelectric layer 126is additionally patterned to define windows 119 that provide access toadditional parts of the surface of the fill material.

In an embodiment, the piezoelectric material deposited to formpiezoelectric layer 126 was aluminum nitride and was deposited with athickness of about 780 nm by sputtering. The piezoelectric material waspatterned by wet etching in potassium hydroxide or by chlorine-based dryetching. Alternative materials for piezoelectric layer 126 include zincoxide and lead zirconium titanate.

A layer of metal is deposited and is patterned to define electrode 124,a bonding pad 138 and an electrical trace 139 extending from electrode124 to bonding pad 138, as shown in FIGS. 7I and 7S.

In an embodiment, the metal deposited to form electrode 124 wasmolybdenum. The molybdenum was deposited with a thickness of about 440nm by sputtering, and was patterned by dry etching. Other refractorymetals such may alternatively be used as the material of electrode 124,bonding pad 138 and trace 139. The electrode, bonding pad and trace mayalternatively comprise layers of more than one material.

The wafer is then isotropically wet etched to remove fill material 105from cavity 104. As noted above, portions of the surface of fillmaterial 105 remain exposed through, for example, windows 119. The etchprocess leaves band-pass filter 100 suspended over cavity 104, as shownin FIGS. 7J and 7T.

In an embodiment, the etchant used to remove fill material 105 wasdilute hydrofluoric acid.

A gold protective layer is deposited on the exposed surfaces of bondingpads 132, 134 and 138.

The wafer is then divided into individual band-pass filters, includingband-pass filter 100. Each band-pass filter is mounted in a package andelectrical connections are made between bonding pads 132, 134 and 138 ofthe band-pass filter and pads that are part of the package.

An embodiment in which acoustic decoupler 130 incorporates a Braggstructure, as shown in FIG. 5D, is made by a process similar to thatdescribed above. The process differs as follows:

After a layer of piezoelectric material is deposited and patterned toform piezoelectric layer 116, a layer of metal is deposited and ispatterned to define high acoustic impedance Bragg element 165 shown inFIG. 5D, bonding pad 134 and electrical trace 135 extending between highacoustic impedance Bragg element 165 and bonding pad 134, in a mannersimilar to that shown in FIGS. 7E and 7O. The layer of metal isdeposited with a nominal thickness equal to an odd, integral multiple ofone quarter of the wavelength in the metal of an acoustic wave having afrequency equal to the center frequency of band-pass filter 100. Highacoustic impedance Bragg element 165 additionally serves as electrode114 as shown in FIG. 5D.

In an embodiment, the metal deposited to form high acoustic impedanceBragg element 165 is molybdenum. The molybdenum is deposited with athickness of about 820 nm (one-quarter wavelength in Mo) by sputtering,and is patterned by dry etching. Other refractory metals mayalternatively be used as the material of high acoustic impedance Braggelement 165, bonding pad 134 and trace 135. The high acoustic impedanceBragg element, bonding pad and trace may alternatively comprise layersof more than one metal.

A layer of low acoustic impedance material is then deposited and ispatterned to define low acoustic impedance Bragg element 163 in a mannersimilar to that shown in FIGS. 7F and 7P. The layer of low acousticimpedance material is deposited with a nominal thickness equal to anodd, integral multiple of one quarter of the wavelength in the materialof an acoustic wave having a frequency equal to the center frequency ofband-pass filter 100. Low acoustic impedance Bragg element 163 is shapedto cover at least high acoustic impedance Bragg element 165, and isadditionally shaped to expose part of the surface of fill material 105and bonding pads 132 and 134. The layer of low acoustic impedancematerial is additionally patterned to define windows 119 that provideaccess to additional parts of the surface of the fill material.

In an embodiment, the low acoustic impedance material is SiO₂ with athickness of about 790 nm. The SiO₂ is deposited by sputtering, and ispatterned by etching. Other low acoustic impedance material that can beused as the material of low acoustic impedance Bragg element includephosphosilicate glass (PSG), titanium dioxide and magnesium fluoride.The low acoustic impedance material can alternatively be deposited bymethods other than sputtering.

A layer of metal is deposited and is patterned to define high acousticimpedance Bragg element 167 shown in FIG. 5D and electrical trace 137extending from high acoustic impedance Bragg element 167 to bonding pad134 in a manner similar to that shown in FIGS. 7G and 7Q. Bonding pad134 is also electrically connected to high acoustic impedance Braggelement 167 by trace 135. The layer of metal is deposited with a nominalthickness equal to an odd, integral multiple of one quarter of thewavelength in the metal of an acoustic wave having a frequency equal tothe center frequency of band-pass filter 100. High acoustic impedanceBragg element 167 additionally serves as electrode 122 as shown in FIG.5D.

In an embodiment, the metal deposited to form high acoustic impedanceBragg element 167 and electrical trace 137 is molybdenum. The molybdenumis deposited with a thickness of about 820 nm (one-quarter wavelength inMo) by sputtering, and is patterned by dry etching. Other refractorymetals may alternatively be used as the material of high acousticimpedance Bragg element 167 and trace 137. The high acoustic impedanceBragg element and the trace may alternatively comprise layers of morethan one material.

A layer of piezoelectric material is then deposited and is patterned todefine piezoelectric layer 126, as described above with reference toFIGS. 7H and 7R, and the process continues as described above tocomplete fabrication of band-pass filter 100.

Band-pass filter 100 is used as follows. Bonding pad 134 electricallyconnected to electrodes 114 and 122 provides a ground terminal of theband-pass filter 100, bonding pad 132 electrically connected toelectrode 112 provides an input terminal of the band-pass filter 100,and bonding pad 138 electrically connected to electrode 124 provides anoutput terminal of the band-pass filter 100. The input terminal and theoutput terminal can be interchanged.

As noted above, band-pass filter 100 may additionally provide electricalisolation between input and output. In such an embodiment, an additionalbonding pad (not shown) is defined in the metal in which electrode 122and trace 137 are defined, and trace 137 extends from electrode 122 tothe additional boding pad instead of to bonding pad 134. Bonding pad 132and 134 electrically connected to electrodes 112 and 114, respectively,provide a pair of input terminals and the additional bonding pad (notshown) electrically connected by trace 137 to electrode 122 and bondingpad 138 electrically connected to electrode 124 provide a pair of outputterminals. The input terminals and the output terminals are electricallyisolated from one another. Again, the input terminals and outputterminals may be interchanged.

A comparison of FIG. 6 with curve 46 of FIG. 4 shows that the slope ofthe out-of-band frequency response of band-pass filter 100 is less steepthan that of the band-pass ladder filter 10 shown in FIG. 1. Thecomparison also shows that, unlike that of band-pass ladder filter 10,the frequency response of band-pass filter 100 does not rise again afterthe initial sharp fall. FIG. 8 is a schematic drawing of an exemplaryembodiment 200 of a band-pass filter in accordance with the inventionhaving an out-of-band frequency response that has a steeper slope thanthat of band-pass filter 100, and in which, after the initial sharpfall, the frequency response rises to a substantially lower level thanthe band-pass ladder filter shown in FIG. 1.

Band-pass filter 200 is composed of a simplified FBAR-based ladderfilter 210 connected in series with band-pass filter 100 described abovewith reference to FIGS. 5A and 5B. Ladder filter 210 is composed ofseries FBARs 212 and 214 and a shunt FBAR 216. Series FBARs 212 and 214have a higher resonant frequency than shunt FBAR 214.

In band-pass filter 100, electrode 112 is connected to ground,electrodes 114 and 122 are connected to the output of ladder filter 210,i.e., to the electrode 218 of FBAR 214, and electrode 124 provides theoutput terminal of band-pass filter 200: FBARs 212, 214 and 216 andband-pass filter 100 are structured so that band-pass filter 100 has abroader pass band than ladder filter 210.

FIG. 9 is a graph showing the calculated frequency response of band-passfilter 200 (curve 242) and that of band-pass filter 100 shown in FIGS.5A and 5B (curve 244). The graph shows that the out-of-band frequencyresponse of band-pass filter 200 has a steeper slope than that ofband-pass filter 100, and the level to which the frequency responserises after the initial sharp fall is lower than that of the band-pasladder filter shown in FIG. 4.

As disclosed in above-mentioned U.S. Pat. No. 6,262,637, the FBARsconstituting an FBAR-based ladder filter are typically all fabricatedusing a common layer of piezoelectric material. Band-pass filter 200 canbe fabricated in a similar way. FBAR 110 (FIG. 5B) of band-pass filter100 is fabricated using the same layer of piezoelectric material asFBARs 212, 214 and 216. Electrode 112 of FBAR 110 is part of the samemetal layer as the electrode 220 of FBAR 216. Electrode 114 of FBAR 110is part of the same metal layer as the electrode 218 of FBAR 214. Afterfabrication of FBARs 110, 212, 214 and 216, a layer of acousticdecoupling material is deposited and is patterned to define acousticdecoupler 130 on electrode 114. FBAR 120 is then fabricated on theacoustic decoupler using a process similar to that described above.

Electrical connections to the electrode 222 of FBAR 212 and to theelectrode 220 of FBAR 216 provide the input terminals of band-passfilter 200 while electrical connections to electrodes 112 and 124 ofband-pass filter 100 provide the output terminals of band-pass filter200.

This disclosure describes the invention in detail using illustrativeembodiments. However, the invention defined by the appended claims isnot limited to the precise embodiments described.

1. A band-pass filter, comprising: a stacked pair of film bulk acousticresonators (FBARs), each of the FBARs comprising opposed planarelectrodes and a layer of piezoelectric material between the electrodes;and an acoustic decoupler between the FBARs.
 2. The band-pass filter ofclaim 1, in which the acoustic decoupler is structured to providesubstantially critical coupling of acoustic energy between the FBARs. 3.The band-pass filter of claim 1, in which the acoustic decouplercomprises a layer of acoustic decoupling material.
 4. The band-passfilter of claim 3, in which: the piezoelectric material has an acousticimpedance; and the acoustic decoupling material has an acousticimpedance less than the acoustic impedance of the piezoelectricmaterial.
 5. The band-pass filter of claim 3, in which: thepiezoelectric material has an acoustic impedance; and the acousticdecoupling material has an acoustic impedance intermediate between theacoustic impedance of the piezoelectric material and the acousticimpedance of air.
 6. The band-pass filter of claim 3, in which theacoustic decoupling material has an acoustic impedance in the range fromabout 2 Mrayl to about 16 Mrayl.
 7. The band-pass filter of claim 3, inwhich the acoustic decoupling material comprises plastic.
 8. Theband-pass filter of claim 3, in which the acoustic decoupling materialcomprises polyimide.
 9. The band-pass filter of claim 3, in which theacoustic decoupling material comprises poly(para-xylylene).
 10. Theband-pass filter of claim 3, in which: the band-pass filter ischaracterized by a center frequency; and the layer of acousticdecoupling material has a nominal thickness equal to an odd integralmultiple of one quarter of the wavelength in the acoustic decouplingmaterial of an acoustic wave having a frequency equal to the centerfrequency.
 11. The band-pass filter of claim 10, in which the acousticdecoupling material comprises plastic.
 12. The band-pass filter of claim10, in which the acoustic decoupling material comprises polyimide. 13.The band-pass filter of claim 10, in which the acoustic decouplingmaterial comprises poly(para-xylylene).
 14. The band-pass filter ofclaim 3, in which: the band-pass filter is characterized by a centerfrequency; and the layer of acoustic decoupling material has a nominalthickness equal to one quarter of the wavelength in the acousticdecoupling material of an acoustic wave having a frequency equal to thecenter frequency.
 15. The band-pass filter of claim 1, in which theacoustic decoupler comprises a Bragg structure.
 16. The band-pass filterof claim 15, in which the Bragg structure comprises one or more lowacoustic impedance Bragg elements interleaved with high acousticimpedance Bragg elements.
 17. The band-pass filter of claim 16, in whichtwo of the high acoustic impedance Bragg elements additionally serve asone of the electrodes of each of the FBARs.
 18. The band-pass filter ofclaim 16, in which: the band-pass filter is characterized by a centerfrequency; and each of the Bragg elements comprises a layer having anominal thickness equal to an odd integral multiple of one quarter ofthe wavelength in the respective material of an acoustic wave having afrequency equal to the center frequency.
 19. The band-pass filter ofclaim 1, additionally comprising an electrical connection betweenadjacent ones of the electrodes of the FBARs.
 20. The band-pass filterof claim 19, in which the acoustic decoupler is located between theadjacent ones of the electrodes.
 21. The band-pass filter of claim 1,additionally comprising a ladder filter electrically connected in serieswith the stacked pair of FBARs.
 22. The band-pass filter of claim 21, inwhich the ladder filter comprises additional FBARs.
 23. The band-passfilter of claim 21, in which: the band-pass filter additionallycomprises an electrical connection between adjacent ones of theelectrodes of the stacked pair of FBARs and the ladder filter; and theremaining ones of the electrodes of the stacked pair of FBARs providethe output terminals of the band-pass filter.
 24. A band-pass filtercharacterized by a center frequency, the band-pass filter comprising: astacked pair of film bulk acoustic resonators (FBARs), each of the FBARscomprising opposed planar electrodes and a layer of piezoelectricmaterial between the electrodes, the piezoelectric material having anacoustic impedance; and between the FBARs, a layer of acousticdecoupling material having a nominal thickness equal to an odd integralmultiple of one quarter of the wavelength in the acoustic decouplingmaterial of an acoustic wave having a frequency equal to the centerfrequency, the acoustic decoupling material having an acoustic impedanceless than the acoustic impedance of the piezoelectric material.
 25. Theband-pass filter of claim 24, in which the acoustic decoupling materialcomprises one of polyimide and poly(para-xylylene).
 26. An electricalfiltering method, comprising: providing a pair of film bulk acousticresonators (FBARs); applying an input electrical signal to one of theFBARs; coupling less acoustic energy between the FBARs than would becoupled by direct contact between the FBARs; and outputting a filteredoutput electrical signal from the other of the FBARs.
 27. The filteringmethod of claim 26, in which: the coupling establishes a first passbandwidth; and the method additionally comprises, prior to the applying,filtering the input electrical signal with a second pass bandwidthnarrower than the first pass bandwidth.
 28. A method of fabricating anacoustically-coupled device, the method comprising: fabricating a firstfilm bulk acoustic resonator (FBAR); fabricating an acoustic decoupleron the first FBAR; fabricating a second FBAR on the acoustic decoupler,including subjecting the acoustic decoupler to a maximum temperature;and prior to fabricating the second FBAR, baking the first FBAR and theacoustic decoupler at a temperature not lower than the maximumtemperature.
 29. The method of claim 28, in which the acoustic decouplercomprises a layer of polyimide.
 30. The method of claim 29, in whichfabricating the acoustic decoupler comprises depositing the layer ofpolyimide by spin coating.